Referring to FIG. 1, a conventional twin-spooled, contra-rotating propeller gas turbine engine, e.g. a propfan engine, is generally indicated at 10 and has a principal rotational axis 9. The term “propfan” will be understood by the skilled person to refer to a gas turbine engine having an open rotor, i.e. having a rotor comprising blades that are not surrounded by a nacelle. The engine 10 comprises a core engine 11 having, in axial flow series, an air intake 12, a low pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, a low pressure turbine 18, a free power (or low-pressure) turbine 19 and a core exhaust nozzle 20. A nacelle 21 generally surrounds the core engine 11 and defines the intake 12 and nozzle 20 and a core exhaust duct 22. The engine 10 also comprises two contra-rotating propeller stages 23, 24 attached to and driven by the free power turbine 19 via shaft 26. The configuration having the propeller stages 23, 24 towards the rear of the gas turbine engine 10 is termed a “pusher” configuration, as opposed to the “puller” or “tractor” configuration having the propeller stages 23, 24 towards the front of the engine 10.
The gas turbine engine 10 works in a conventional manner so that air entering the intake 12 is accelerated and compressed by the low pressure compressor 14 and directed into the high-pressure compressor 15 where further compression takes place. The compressed air exhausted from the high-pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high-pressure, low pressure and free power turbines 17, 18, 19 before being exhausted through the nozzle 20 to provide some propulsive thrust. The high-pressure, intermediate pressure and free power turbines 17, 18, 19 respectively drive the high and low pressure compressors 15, 14 and the propellers 23, 24 by suitable interconnecting shafts. The propellers 23, 24 normally provide the majority of the propulsive thrust. In the embodiments herein described the propellers 23, 24 rotate in opposite senses so that one rotates clockwise and the other anti-clockwise around the engine's rotational axis 9.
One problem with a conventional pusher propeller gas turbine engine 10 is that its cruise speed is limited to slightly below transonic, predominantly due to the drag rise encountered when flying at higher speeds. One of the main causes of this drag rise is that generally the root of each blade forming the propeller stages 23, 24 can not be shaped with the thin profiles required for high speed. The root has to be thick enough to guarantee the structural robustness of the blades given the high aerodynamic and mechanical loads acting on the propeller stages 23, 24, which disadvantageously adds significant weight to the engine 10. The airflow passing between the blade roots may easily become supersonic if the propeller gas turbine engine 10 operates at transonic cruise speed, around Mach 0.8. This results in disadvantageous increased noise, aerodynamic losses and possible mechanical excitation, phenomena which it is desirable to avoid or at least limit.
The example shown in FIG. 1 has a nacelle 21 with a conventional fore-body extending from the intake 12 to a point at which the nacelle diameter is at a maximum. In an alternative configuration, for example, the General Electric GE36 Unducted Fan, there is then a reduction in the nacelle diameter downstream of the maximum diameter point and ahead of the first propeller stage. In effect the geometry results in a diffusion upstream of the first propeller stage such that the flow velocity is reduced. The annulus line through the propeller stages of the GE36 is then close to cylindrical and is followed by a short curved after-body to close out at the core exhaust nozzle. By diffusing the flow ahead of the first propeller stage, the Mach number of the flow at the hub of the GE36 may be reduced and the rotor efficiency may benefit as a result. However, due to the large maximum nacelle diameter of the GE36 required to achieve the diffusion, a large free-stream over-speed over the outer span of the propeller blades occurs and this is detrimental to the rotor efficiency.
The present disclosure therefore seeks to address these issues.